Mobile electronic devices which are capable of music playback have become very popular recently, with the IPOD™ by Apple® Computer Inc. being one of the most prominent examples. These music players are mainly intended for being used in conjunction with head- or earphones. For a typical mobile use of such devices this is of course the most preferable listening manner. However these players have become rather sophisticated recently, many of them even comprise hard disks capable of storing gigabytes of music. Therefore it seems only logical that a user would want to use his player, whereon a big amount or even all of his own music is stored, as the source of music data also in other environments, and also for playing back music with normal speakers.
Of course he could simply use the earphone or line out output to connect his player device with his hi-fi equipment or the like. However cable connections are inconvenient, particularly in conjunction with small mobile player devices. Because of the wide variety of used plug/socket connector systems this is also likely to cause incompatibilities between devices.
Another example would be to use such a player as a replacement for a CD-changer in a vehicle. However car radio systems usually do not comprise any input interface to connect a mobile player. As one of the main advantages of mobile music players is the possibility to easily carry it along it would be desirable if it could be coupled with other equipment in a standardized way, with some kind of common interface.
Therefore it is apparent that a wireless transmission of music data from the player is most desirable. As many audio playback devices like stereo systems and car radios comprise an FM tuner or receiver a known implementation of such wireless transmission is to “mimic” the music player as a conventional FM radio station and to send the audio data encoded as a standard FM radio transmission.
In the United States of America (and also other countries) the FCC (Federal Communications Commission) allows the usage of unlicensed (i.e. personal/private) FM-radio transmitters according to FCC rule 15 (see section 15.239). Such a transmitter can thus be used for conveniently transmitting sound or music from any device wirelessly to an FM radio operating in the 88-108 MHz band, e.g. from a CD-player or an MP3-player. This allows listening to music from such a device e.g. through a car FM radio. Due to the restricted transmission power with a field strength of 250 μV/m in a distance of 3 meters the transmission range of such private transmitters is small. Interference is therefore expected to be low. However, interference with licensed FM transmitters, e.g. a radio station, is usually not allowed.
Conventional transmitters for that purpose simply transmit on a fixed frequency or frequency that can manually be chosen. This requires manually setting the corresponding transmission frequency on both the transmitter connected with the player device and the FM radio receiver. As described above an interference with a licensed transmitter is usually not allowed, so the user has to perform a manual search for free frequencies before setting the frequency in order not to violate that regulation. This is of course very inconvenient. Additionally, when the FM receiver is a car radio and the user is driving, the situation related to free frequencies will change over time. This will certainly require re-tuning from time to time, e.g. because a licensed transmitter (e.g. a radio station) will come in range that is transmitting on the same frequency as the unlicensed private transmitter of the user. On the one hand this is normally prohibited as discussed above and should therefore not occur, and on the other hand such interference will most severely decrease the quality of the signal received from the music player, as the transmission power of the private transmitter is substantially lower than that of licensed transmitters. Manual re-tuning will thus be unavoidable to maintain the quality of the transmission of audio data. However manual re-tuning while driving is to be avoided in order not to affect driving safety.
Therefore it is an object of the present invention to provide a method and a device for wirelessly transmitting audio data from an audio playback device to an FM radio receiver, wherein manual re-tuning on the side of the FM radio receiver (and in advanced embodiments even at the transmitter) is not necessary while at the same time the transmission is maintained at the maximum possible quality and is also made more reliable than with the prior art.
The Radio Data System (RDS) is an extension of the standard FM radio transmission. It allows FM broadcasters to send more than just an analog audio signal out over the air interface. Stations can transmit digital RDS data for reception by RDS-equipped FM tuners. This technology opens up a whole new range of services to a listener with RDS reception capability. The RDS system itself is not part of the present invention and shall thus not be described in detail here. Detailed information about the function of RDS can be obtained from many sources.
One special feature of RDS is the so-called Alternate Frequency (AF) function. AF, which should better be referred to as Alternate Frequency Switching, automatically returns an FM tuner to the strongest signal carrying the program a user was originally listening to when the original broadcast got too weak to be received properly. The way this function operates can be summarized as follows:
The original RDS broadcast contains a coded list of all the alternate frequencies carrying the same information, i.e. being used for broadcasting the same program. When the original broadcast fades below a certain threshold the RDS circuitry will search the alternate frequencies for the strongest, most useable signal and automatically switch to it without any action required by the listener.
The most prominent use case is a moving FM radio receiver, e.g. a car stereo system. During driving the moving receiver will occasionally leave a reception area of a certain station/program transmitting on a certain frequency. If that station is part of a network broadcasting the program, with a number of transmitting stations with different geographical location and each with its own frequency for the same program, the AF feature of RDS enables the receiver to tune to the best station for that specific program at all times. That is, the FM tuner is automatically tuned to the frequency of the station providing the best reception of the same program.